For those of us in the northern hemisphere, today is the longest day of the year. Here in Oakland, California, we’re soaking up over 15 hours of sunlight. Globally, about 7600 exajoules of solar energy—that is, about 15 times the amount of energy humanity will use this year—will reach the earth’s surface in the next 24 hours, powering everything from phytoplankton to redwoods to backyard BBQs.

It is, in other words, a beautiful day to get outside and celebrate the power plant that has kept our planet humming with life for the last 3.7 billion odd years.

It’s also the perfect day to celebrate what we’re learning to do with the sun’s power. The past few years have ushered in an unprecedented, unforeseen, and largely unheralded solar energy revolution. As recently as 2005, global installed solar power capacity stood at 4.5 gigawatts (GW). Today, the figure exceeds 65 GW, which is equivalent to the capacity of about 130 average-sized coal-fired power plants.

To put recent growth of solar power in perspective it helps to look at how it has played out in particular places. Take the U.S., for example. Solar is America’s fastest growing industry, and already employs more than 100,000 men and women—more than U.S. steel production and more than U.S. coal mining. In California, which leads the nation on solar power, the number of installed solar energy systems has increased from about 500 in 1999 to more than 50,000 in 2011. These days, when you fly into a place like Oakland, you can see your plane reflected in the rooftops below.

Then there’s Germany. A few weeks ago, every energy wonk in the world did a double take after learning that the country had met a third of its weekday noontime electricity demand—and half of noontime electricity demand on a Saturday—with solar power. These statistics are amazing in of themselves, but even more amazing is the fact that three-quarters of Germany’s solar energy capacity is locally-owned. Put simply: the world’s fourth largest economy and seventh largest energy consumer is now meeting a huge chunk of its electricity demand via clean energy sources whose economic benefits flow to everyday people.

So what’s driving the solar revolution?

Better, cheaper solar technology is a big part of it. While the efficiency with which solar cells convert sunlight to usable energy has improved dramatically over the last few decades, manufacturing costs have come down. The upshot of both these trends is that the cost per watt for solar panels declined from $22 in 1980 to well under $1 by January of this year.

Policy has also played an important role. Many countries, Germany included, have implemented feed-in tariffs that essentially subsidize solar and other forms of renewable energy. In the U.S., government support for renewable energy has been more scattered, but state and federal incentive programs have still played a critical role in expanding the solar industry. For a fraction (one twelfth, to be precise) of the government dollars that go to the fossil fuel industry, these programs are driving innovation, bringing us closer to the day when solar will be cheaper than coal, even absent incentives. In California, we are already arriving at that point.

Finally, a new breed of solar entrepreneur is bringing down the costs associated with installing and financing solar power. One major innovation came from Jigar Shah, a founder of SunEdison, who realized that it might make more sense for many property owners to lease, rather than purchase, solar energy systems for their roofs. Thanks to solar leases, building owners are now able to go solar with no upfront cost and make lease monthly payments that are typically less than what they would pay the utility for the same amount of energy. More recently, innovations in cleantech have been merging with innovations on the net to form what Sunil Paul calls the “cleanweb.” Sungevity uses advanced software and satellite imagery to provide potential customers with iQuotes for solar installations on their roofs. My own company, Solar Mosaic, is using the web to empower people to pool their resources to create solar in their communities. Our efforts recently won us a $2 million grant from the Department of Energy’s Sunshot Inititiave, which aims to catalyze a dramatic decrease in solar energy costs over the next decade.

There are plenty of days in the year to think about the mistakes we’ve made fighting climate change and how much we have left to do to create a society powered by clean energy. Yet, for now, with the sun shining outside, it’s worthwhile to step back and think about what we have accomplished. It’s worthwhile to think about all of the solar panels that are out there catching some serious rays today and how much coal–about 186 million tons per year–they’ll keep in the ground.

What comes next? According to McKinsey and Co, the revolution is just getting started: we’re on track to reach 400-600 GW of global solar capacity (that is, the equivalent of between 800 and 1200 coal-fired power plants) by 2020. Other projections estimate that by the end of the decade solar will be the cheapest power source for more than 80 million Americans. Some would say these projections are overly optimistic. I suspect we can do better than the projections suggest. We’ll have to work hard. We’ll have to innovate like crazy. We’ll have to beat some seriously big and bad opponents and overcome some immense challenges.

Fortunately, we know can do all of this, because we’re already doing it.

This post was crossposted from the Huffington Post.